The invention relates generally to level gauging, and more specifically the invention relates to a method and an apparatus for radar-based level gauging.
Radar-based methods are extensively used for level gauging, i.e. measuring a distance from the top of a tank to a surface of a liquid or some kind of granular solid stored in the tank, by means of transmitting microwaves towards the surface of the liquid or the granular solid, receiving the microwaves as reflected against the surface of the liquid or solid, i.e. the surface echo, and calculating the level of the liquid or solid in the tank from the propagation time of the transmitted and reflected microwaves.
One very general problem in this respect is that the tank includes typically various structures, such as beams, support beams, pipes, agitators, tank walls, etc. Such structures may also reflect microwaves, and such echoes can interfere with the microwaves reflected from the surface of the liquid or solid, the level of which being gauged. An interfering echo having a signal strength considerably lower than the signal strength of the surface echo will cause measuring errors if the structures creating the interfering echoes is close to, i.e. within a few tenths of a meter from, the surface. If the interfering echo is stronger than the echo from the surface the interfering echo may falsely be selected as the surface echo. Various echo logic methods have been applied to decrease this risk, but nevertheless this is a large problem in many tank environments.
The selection of correct microwaves is thus extremely important and any possibility of distinguishing microwave signals reflected from the surface of the liquid or solid from microwaves reflected from other structures is very useful.
Typically, prior art radar level gauges typically select the strongest echo.
The ideal case is to use an antenna with a rather narrow lobe located in the tank where no disturbing echoes are close to the antenna lobe. In this case the surface echo may be the strongest one even after some degradations due to turbulence, foam etc. For smaller tanks various tank structures may approach the antenna beam, not at least since the antenna has to be smaller. Furthermore when the echo from the surface is close to a disturbing echo there is a risk for a large measuring error.
A main object of the invention is thus to provide a method and an apparatus for radar-based level gauging, wherein detected microwaves as reflected from the surface of the liquid or solid can be distinguished from detected microwaves as reflected from other disturbing structures.
In this respect there is a particular object of the invention to provide such a method and such an apparatus, which are very useful in tanks having a large number of disturbing structures and in tanks where the radar-based level gauging equipment has to be mounted in region where disturbing structures do occur.
A further object of the present invention is to provide such a method and such an apparatus, which provide for level gauging also of highly turbulent surfaces, where the reflected microwaves are weak.
A still further object of the invention is to provide such a method and such an apparatus, which are reliable, efficient, accurate, and precise.
These objects, among others, are attained by methods and apparatuses as claimed in the appended claims.
According to a first aspect of the present invention there is provided a method for radar-based gauging of the level of a substance, e.g. a liquid or a granular solid, in a tank having one or several interfering structures, such as e.g. a beam, a support beam, an agitator, or a tank side wall. The method comprises to transmit a microwave signal in a predetermined polarization state, e.g. left hand circular polarization, towards a surface of the liquid or granular solid and the interfering structure(s). Microwave signals as reflected against the surface of the liquid or granular solid and against the interfering structure(s) are detected temporally resolved and separately in at least two different polarization states, e.g. left and right hand circular polarizations. Then, the detected microwave signal, which has been reflected against the surface of the liquid or granular solid, is distinguished based on signal strengths of the microwave signals detected temporally resolved and separately in the two different polarization states. Finally, the level of the liquid or granular solid in the tank is calculated based on a propagation time of the distinguished microwave signal.
The inventor of the present invention has noted that the detected microwave signal, which has been reflected against the surface of the liquid or granular solid, is distinguishable by means having notably different signal strength in the two different polarization states. This is true as long as the surface is calm. In contrast thereto, the detected microwave signal(s), which has been reflected against the surface of the interfering structure(s), has (have) typically similar signal strength(s) in the two different polarization states. For the microwave signal any two preferably orthogonal polarizations will cover all different possible combinations. In the signal processing, however, more than two different signals may be formed and used.
By detecting the microwave echoes in two separate polarizations a further processing can be performed not only limited to the detection and rejection of disturbing echoes, but also for enabling a decrease of the influence of disturbing echoes close to the surface echo. For each echo a linear combination of the signals in the two received polarizations can be found, where the echo is very weak, while other echoes are less or much less reduced. This enables in many cases a substantial improvement of the signal-to-disturbance ratio assuming that different linear combinations are used for different disturbing echoes. Using prior art equipment large measuring errors cannot be avoided if the disturbing echo is close to the surface echo and of comparable strength.
Further, a time variation of the signal strengths may be recorded to distinguish a microwave signal reflected against a turbulent surface from microwave signals reflected against fixed interfering structures. The time variation for the echo from a turbulent surface can in stochastic sense in most cases be described as Raleigh distributed. This can be used as one distinctive feature to separate the echo from the turbulent surface from the much more steady echo from a fixed disturbing echo. In one embodiment of the invention the polarization diversity is combined with the time variation measurement to obtain a selection criteria, which is usable, both for calm and turbulent surfaces.
Transceiver apparatuses capable of producing polarized microwave radiation and of receiving reflected microwave radiation in two different polarization states separately include preferably any of a power divider, particularly a Wilkinson power divider, a directional coupler, a ferrite circulator, or multiple antennas.
According to a second aspect of the present invention there is provided a radar-based level gauge apparatus for performing the method according to the first aspect of the invention.
By means of the present invention a very robust routine for distinguishing detected microwave signals, which have been reflected against the surface of the substance gauged, may be implemented. As compared to prior art devices the invention provides for measurement in more disturbing environments, i.e. where more interfering echoes do occur, with higher accuracy. For instance, microwave signals reflected at the surface of the substance, may be distinguished despite being weaker to much weaker than a microwave signal as reflected against an interfering structure.
The radar-based level gauges are used to measure levels in tanks, which for the purpose of the present invention include not only large containers but also processing apparatuses such as, for example, reactors, centrifuges, mixers, hoppers, graders, or heat-treatment furnaces and similar devices, which are used in e.g. food chemistry, pharmaceutical chemistry, biochemistry, gene chemistry and petrochemistry.